Thermal treatment of electromelted refractory materials

ABSTRACT

A process is disclosed for imparting an increased corrosion resistance to electromelted refractory materials of the AZS type (alumina, zirconia, silica) comprising crystalline phases and one vitreous phase, the process being characterized in that the refractory material is subjected to a controlled thermal treatment at temperatures between 1300*C and 1600*C. The starting material has a chemical composition, expressed in oxides, within the following weight ranges: Al2O3 45-65%, ZrO2 10-40%, SiO2 12-20%, Na2O 0.8-1.4%, corresponding to the following weight ranges of the individual phases: Corundum 30-60%, Baddeleyite 10-40%, Mullite 0-40%, vitreous phase 15-25%.

United States Patent 1191 Cevales Aug. 28, 1973 THERMAL TREATMENT OFELECTROMELTED REFRACTORY MATERIALS Inventor:

Foreign Application Priority Data July 13, 1970 Italy 27345 A/70 US. Cl.106/57, 106/65 Int. Cl. C04b 35/18, C04b 35/48 Field of Search 106/57,65

References Cited UNITED STATES PATENTS FOREIGN PATENTS OR APPLICATIONSl,9l4,982 lO/l969 Germany 106/57 Primary Examiner-James E. PoerAnorneyStevens, Davis, Miller & Mosher [5 7] ABSTRACT A process isdisclosed for imparting an increased corrosion resistance toelectromelted refractory materials of the AZS type (alumina, zirconia,silica) comprising crystalline phases and one vitreous phase. theprocess being characterized in that the refractory material is subjectedto a controlled thermal treatment at temperatures between 1300C andl600C. The starting material has a chemical composition, expressed inoxides. within the following weight ranges: AI,O;, 45-65%, ZrO lO-40%,SiO, l2-20%, Na,0 08-14%, corresponding to the following weight rangesof the individual phases: Corundum 30-60%, Baddeleyite 1040%, Mullite0-40%, vitreous phase 15-25%.

4 Claims, No Drawings THERMAL TREATMENT OF ELECTROMELTED REFRACTORYMATERIALS The present invention relates to a thermal treatment ofelectromelted refractory materials, and more particularly, to a thermaltreatment of electromelted refractory materials having a vitreous phase.

It is well known that during the cooling off of molten refractorymaterial, the solid material thus obtained turns out to be composed ofphases that may be in conditions of metastability and that,consequently, these phases may be modified when subjected to furthertreatments.

It is also well known that when such solid materials are used as alining for glass-melting furnaces or for furnaces for heating up steelingots-by reason of which they come into contact with molten glass orwith slagthe vitreous phase plays an important role in the corrosionphenomena.

Thus, a principal object of this invention is that of providing a methodfor reducing the vitreous phase of the electromelted material so as torender same more corrosion resistant.

Another object of the invention is that of producing in the startingmaterial permanent modifications, not so much in the chemicalcomposition as rather in the relationships between the crystalline andthe vitreous phases, in order to obtain an end product that will have asubstantially improved resistance to corrosion.

These as well as other objects are attained by the method of the presentinvention, which consists essentially in subjecting the refractorymaterial to a controlled thermal treatment, operating at temperaturesbetween 1,300" and 1,600C, but preferably between 1,500 and 1,600C.

The thermal treatment may be carried out, instead of on already-formedrefractory material, also on material that is still in an intermediateprocessing stage, and more particularly after its having been pouredinto the mold and after having allowed a sufficient lapse of time forthe formation of a first solid shell. In order that the phasesconstituting the starting material may be in a condition to undergomodifications and attain conditions of stable equilibrium, it isnecessary that the thermal treatment be prolonged for a period of timebetween 8 and 12 days and, speaking generally, about 10 days.

Starting materials that prove to be particularly suited for the presentpurpose are the electromelted refractory materials of. the AZS type(alumina, zirconia, silica) having a vitreous phase and a chemicalcomposition, expressed as oxides, falling within the following ranges:A1 45-65%, ZrO, -40%, SiO, 12-20%, Na O 0.8-1.4%, corresponding to thefollowing ranges of the single phases: corundum 30-60%, baddeleyite10-40%, mullite 0-40%, and vitreous phase 15-25%. All percentages hereand elsewhere are by weight.

After thermal treatment, one obtains a product the individual phases ofwhich fall, according to the treatment temperatures, within thefollowing ranges:

Treatment Temperatures The vitreous phase may be reduced down to 7percent; also its composition turns out to be modified as will be seenfrom the following tables. The determination of the single phases wascarried out by means of chemical analysis and by diffractometric X-rayanalysis. The method used for the analysis of the vitreous phase was thecommonly used one of extraction with hydrofluoric acid.

For the thermal treatment there may be used any conventional kind offurnace operating with gaseous or liquid fuels, provided that it beequipped with means insuring the desired control of the processingtemperature.

At the end of the residence time in the furnace at the establishedtemperature, the refractory material is left to cool down at an averagerate of temperature drop of 120-l50C per day.

In order still more clearly to illustrate the present invention, thefollowing illustrative nonlimiting examples are given:

EXAMPLE 1 A sample of electromelted Zetacor A refractory, produced byMONTECATINI EDISON, and having the following composition:

1 0 47. 7% ZrO, 36. 1 g 14.7 0 1.0 FqO, 0.06 0.04 0 0.22 0 0.05

was heated in a furnace fired by oil burners, at temperatures between1,300 and 1,600C. The duration of the treatment was 240 hours.

In Table 1 are given the data concerning the ranges of the crystallineand vitreous phases of the starting product as well as the dataconcerning the samples after treatment for 240 hours at 1,300", 1,400,1,500, and 1,600C.

TABLE 1 Product after treatment Starting proat the ,tem rarure of Phasesduct as such 1300C 1400 1500C 1600C Corundum 43 38 29 20 12 Baddeleyite36 35 34 34 34 Mullite 3 15 25 36 46 Vitreous phase l8 l4 l2 l0 8 InTable 1 bis are recorded the compositions of the vitreous phase of thestarting material and of the samples subjected to thermal treatment:

TAB LE 1 bis Vitreous phase Vitreous Phase Extracted extracted on theafier a treatment of 240 hours Compostarting proat the temperaturesindicated nent ducts as such 1300C 1400C 1500C 1600C l: A1 0, 21.9 29.1034.20 42.9 47.6 210, 4.9 6.91 9.07 12.0 15.7 SiO, 67.05 57.10 49.5238.44 30.99 Na.,0 5.45 5.93 6.10 5.20 3.87 Fe,0, 0.21 0.27 0.27 0.350.60 TiO, 0.13 0.11 0.13 0.14 0.24 CaO 0.36 0.58 0.71 0.97 1.00

EX A M P L E 2 Another example of the same material as that used inExample 1 and having the following composition:

is subjected to heating at a temperature between 1,300 and 1,600C for240 hours.

The following Table 2 reports the data concerning the ranges of thecrystalline and vitreous phases of the starting product and of thesamples subjected to thermal treatment, while Table 2 bis shows thecompositions of the corresponding vitreous phases.

TABLE 2 Product after Thermal Treatment Starting Proat; Phases duct asSuch 1300C 1400C 1500C 1600C Corundum 42 38 30 20 12 Baddeleyite 37 3736 35 35 Mullite 2 10 22 35 44 Vitreous Phase 19 15 12 10 9 TABLE 2 blSVitreous phase Vitreous Phase extracted after a extracted on thetreatment of 240 hours at the Compo- Starting protemperatures indicatednents ducts as such 1300C 1400 1500C 1600C A1 23.2 25.3 28.6 35.4 49.6ZrO, 5.6 5.92 7.13 8.30 18.0 SiO, 64.93 61.75 56.68 49.20 27.92 Na O 5.66.14 6.44 5.8 2.75 Fe,0;, 0.17 0.20 0.35 0.29 0.49 T 0.15 0.15 0.13 0.130.19 CaO 0.35 0.54 0.67 0.84 I 1.05

EXAMPLE 3 A sample of ZAC 1681, of the following composition:

A1,0, 50.0 ZrO, 31.5 S10, 16.8 Na,O 1.43 F90, 0.08 T10, 0.06 CaO 0.28MgO 0.05

TABLE 3 Product afier Thermal Treatment Starting Proat Phases t s Such1300C 1400T 1500 C [600C Corundum 45 44 33 22 Baddeleyite 31 3O 29 29 29Mullite 0 10 26 39 47 Vitreous Phase 24 16 12 1O 9 TABLE 3 bis Vitreousphase extracted after a treatment of 240 hours at the temperaturesindicated Vitreous phase extracted on the Compostarting pronents ductsas such 1300C 1400C 1500C 1600C 32 Al ,0, 21.1 22.6 27.6 33.2 42.6 2103.63 4.36 5.54 5.59 11.6 SiO, 68.94 65.98 59.26 52.29 39.51 Na,0 5.606.32 6.65 6.54 4.70 Fe,O, 0.18 0.15 0.21 0.43 0.52 T10, 0.20 0.18 0.190.18 0.19 CaO 0.35 0.41 0.55 0.77 0.88

EXAMPLE 4 A sample of Zetacor A, of the following composit1on:

SiO 15.1

Fe,O; 0.07

was treated for 240 hours at a temperature of 1,450C, following theprocedures described in Example 1. In Table 4 are recorded the datarelating to the ranges of the crystalline and vitreous ranges of thestarting product and of the sample treated at 1,450C. Table 4 hisrecords the compositions of the corresponding vitreous phases.

A sample of Zetacor A, having the following composition:

A1 0; 49.6 Z10, 3 1 .2 S10 16.9 Na o l .26 rep, 0.05 TiO, 0.05 C30 0.12

was treated for 240 hours at a temperature of 1,450C, following theprocedures described in- Example 1. In Table 5 are recorded datarelating to the ranges of the crystalline and vitreous phases of thestarting product and of the samples treated at 1,450C. In Table 5 bisare recorded the corresponding compositions of the vitreous phases.

TABLE 5 Starting Product after Treatment at Phases Product a Temperatureof 1450C Corundum 47 39 Baddeleyite 30 30 Mullite 3 19 Vitreous Phase 2012 TABLE 5 bis Vitreous Vitreous Phase Extracted Compo- Phase Extractedafter treatment for 240 nent on the Starting Product Hours at l450C Al,020.6 32.6 ZrO, 5.7 7.4 S 67.10 51.64 Na,O 5.7 7.4 Fe,0;, 0.33 0.29 T100.24 0121 CaO 0.33 0.46

EXAMPLE 6 A sample of Mecsial C 15, produced by MON- TECATlNl EDI-SON,and having the following composition:

A1 0 63.5 2 10 12.0 $10 20.5 Na,O 1.28 131 0 0.53 T10 1.95 CaO 0.27

was subjected to a thermal treatment for 240 hours at l,350C. In Table 6are recorded all the data relating to the ranges of the crystalline andvitreous phases of the starting product as such and of the sampletreated at l,350C. In Table 6 bis are recorded the correspondingcompositions of the vitreous phase.

TABLE 6 Starting Phases Product Product after Treatment at as such aTemperature of 1350C Corundum 36 29 Baddeleyite 1 1 1 l Mullite 34 45Vitreous phase l9 15 TABLE 6 bis Vitreous Vitreous Compo- PhaseExtracted Phase Extracted nent on the Starting Product after Treatmentfor 240 as such Hours at I350C A1,o 16.1 18.9 ZrO, 1.26 2.02 S10, 72.6768.19 Na,O 4.60 5.78 rep, 2.09 2.32 TiO, 2.95 2.22 CaO 0.33 0.57

What is claimed is: l. A thermal treatment process for improving thecorrosion resistance of fused refractory materials made from a batchcomprising, on the basis of the oxides, 45 to 65 percent by weight of A10 10 to 40 percent by weight of ZrO 12 to 20 percent by weight of SiOand 0.8 to 1.4 percent by weight of Na,O, and constituted initially of30 to 60 percent by weight of corundum, 10 to 40 percent by weight ofbaddeleyite, 0 to 40 percent by weight of mullite as crystalline phasesand 15 to 25 percent by weight of a vitreous phase, said processcomprising maintaining the refractory material at a temperature between1,300 and 1,600C for 8 to 12 days.

2. A process as claimed in claim 1, wherein said thermal treatment iscarried out at a temperature between 1,500 and 1,600C.

3. A process as claimed in claim 1, wherein the duration of the thermaltreatment is about 10 days.

4. A process as claimed in claim 1, wherein the refractory material,after the said thermal treatment, is

slowly cooled at a rate of about to C per day. i i

2. A process as claimed in claim 1, wherein said thermal treatment iscarried out at a temperature between 1,500* and 1, 600*C.
 3. A processas claimed in claim 1, wherein the duration of the thermal treatment isabout 10 days.
 4. A process as claimed in claim 1, wherein therefractory material, after the said thermal treatment, is slowly cooledat a rate of about 120* to 150*C per day.